The present invention relates generally to gas turbine engines, and, more specifically, to rotors or drive shafts therein.
An aircraft gas turbine engine typically includes one or more drive shafts for transferring torque from one rotating component to another. For example, in a turbofan engine, the fan is driven by the low pressure turbine (LPT) by a first drive shaft, and the compressor is driven by the high pressure turbine (HPT) by a second drive shaft disposed concentrically around the first drive shaft.
The drive shafts must be formed of suitable high temperature, high strength materials for carrying loads during operation at elevated temperatures. Both shafts are subject to torsion loads as the turbines drive the corresponding fan and compressor. The pressure forces acting across the fan, compressor, and turbines place the corresponding drive shafts under tension during operation. The drive shafts are also subject to bending and gravity loads as the engine elastically deflects during aircraft movement.
Various forms of composite materials are being developed for use in such shafts for reducing the overall weight of the engine while maintaining suitable strength under the specific operating conditions, for example titanium metal matrix composites. However, shafts constructed from composite materials typically must include monolithic metallic end pieces at the required mechanical joints in the engine such as bolted flange joints or spline joints, which are used to connect the drive shafts to the other components of the engine.
These monolithic end pieces can not be bonded directly to the composite portions of the shaft, because this would result in the formation of intermetallic compounds at the interfaces between the dissimilar materials, causing brittleness and unpredictability in the joint properties. It is known to avoid the formation of intermetallic compounds by using a layered transition piece which incorporates a barrier material such as a niobium alloy.
These transition pieces have relatively lower strengths than the other components of a shaft. Therefore, the transition pieces must be processed in a manner to avoid any damage or weakening thereof, in order to preserve an adequate overall margin of strength in the completed shaft. Furthermore, in order to obtain high strength levels in the monolithic end pieces of the shaft, they must be properly heat treated, without subjecting the metal matrix composite components to excessive temperatures.
Accordingly, it is desired to provide an improved gas turbine engine drive shaft having reduced weight while maintaining stiffness and strength at elevated temperature, and including suitable mechanical joints for connection with adjoining components.
The above-mentioned need is met by the present invention, which provides a method for assembling a shaft having a metal matrix composite mid shaft and monolithic high strength alloy forward and aft shafts. The forward and aft shafts are each inertia friction welded to separate annular transition pieces which include a barrier layer operative to prevent the formation of intermetallic compounds. Special tooling is used to avoid applying shear stresses to the transition piece during welding. The welded forward and aft subassemblies are subsequently heat treated before inertia friction welding them to the mid shaft.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.